The molecular mechanism of cell entry from blood into tissues has been intensely studied over more than three decades. This work has led to a well-defined multistep model of tissue entry by cells and the development of approved therapeutic agents that target entry as a treatment for immunological diseases. Cell egress from tissues into circulation is equally important for immune function yet has been a focus of study for only the last several years. The potential clinical impact of developing agents to modulate cell egress has been highlighted by the September 2010 approval of FTY720 (Fingolimod), a drug that inhibits lymphocyte egress from lymphoid organs, as the first oral treatment for multiple sclerosis. FTY720 is a ligand for four of five sphingosine-1-phosphate (S1P) receptors. T and B cell egress from lymph nodes, spleen and Peyer's patches is dependent on S1P and S1P receptor-1 (S1PR1). This proposal will identify the pathway of B cell egress from the spleen into blood. Secondly, it will characterize how S1PR1 regulates marginal zone B cell movement and antigen transport between subcompartments within the spleen. Thirdly, it will explore the role of a migration- inhibitory S1P receptor, S1PR2, in controlling egress of cells from non-lymphoid tissues into lymph. This work will elucidate key requirements for lymphocyte egress from lymphoid and non-lymphoid tissues, findings that may help in the development of new therapeutics for treatment of autoimmune diseases.